The present invention relates to polymer hydrogels and methods of preparation thereof.
Polymer hydrogels are cross-linked hydrophilic polymers which are capable of absorbing high amounts of water. In particular, cross-linked polymer hydrogels capable of absorbing an amount of water in excess of 10 times their dry weight are defined as “superabsorbent”. Some of these materials are even capable of absorbing over 1 liter of water per gram of dry polymer.
The cross-links or cross-linking knots, i.e. the physical or chemical bonds between the macromolecular chains forming the polymer hydrogel network, guarantee the structural integrity of the polymer-liquid system, on the one hand preventing the complete solubilisation of the polymer, and on the other hand allowing the retention of the aqueous phase within the molecular mesh.
The superabsorbent polymer hydrogels which are currently available on the market are characterised not only by their marked absorbent properties, but also by their biocompatibility, which is probably due to the high water content, and, above all, by the possibility of adjusting their absorption properties according to the external stimuli. Consequently, such polymer hydrogels may be used as intelligent materials, for example for the manufacture of sensors or actuators for a number of industrial applications. Besides the usual applications as absorbent cores in the field of personal hygiene absorbent products, there are more recent and innovative applications such as for example in the biomedical field, for the development of controlled release drug formulations, artificial muscles, sensors, etc., and in agriculture and horticulture, for example in devices for the controlled release of water and nutrients in arid soils.
However, the superabsorbent polymer hydrogels currently available are almost exclusively acrylic-based products, and hence not biodegradable.
Given the growing interest in environmental protection issues, over recent years a vast amount of interest has been focused on the development of superabsorbent materials based on biodegradable polymers, having properties which are similar to those of the traditional superabsorbent polyacrylics.
Examples of biodegradable polymers used to obtain superabsorbent polymer hydrogels are starch and cellulose derivatives.
In 1990 Anbergen and Oppermann [1] proposed a method for the synthesis of a superabsorbent material made entirely from cellulose derivatives. In particular, they used hydroxyethylcellulose (HEC) and a carboxymethylcellulose sodium salt (CMCNa), chemically cross-linked in a basic solution with divinylsulphone. However, the absorption properties of such materials are not high compared to those of the acrylic-based superabsorbent materials.
In 1996 Esposito and co-workers [2], studying the synthetic process proposed by Anbergen and Opperman, developed a method for increasing the absorption properties of the gel, acting mainly on the physical properties of the material. The basic idea was the induction of microporosity into the polymer structure, so as to promote absorption and retention of water by capillarity. Said microporosity was induced during the drying step, which was carried out by phase inversion in a nonsolvent for the polymer, and the absorption properties of the material thus obtained were markedly superior to those of the air-dried gel.
CMCNa may be chemically cross-linked with any reagent which is bifunctional with respect to cellulose. Besides the divinylsulphone used in the synthetic process according to Anbergen and Opperman, epichlorohydrin, formaldehyde and various diepoxides have also been used as cross-linking agents. However, such compounds are highly toxic in their unreacted states [3]. Some carbodiimides are known amongst the unconventional cross-linking agents. Particularly, the use of carbodiimides in order to cross-link salified or non-salified carboxymethylcellulose (CMC) was described in [4]. Carbodiimides induce the formation of ester bonds between cellulose macromolecules without participating in the bonds themselves, instead giving rise to a urea derivative having very low toxicity [5]. A superabsorbent polymer hydrogel obtained by cross-linking carboxymethylcellulose sodium salt and hydroxyethylcellulose with carbodiimide as the cross-linking agent is disclosed in the international patent application WO 2006/070337 [6].
However, the carbodiimide used as a cross-linking agent in WO 2006/070337 has the disadvantage of being extremely expensive. Moreover, during the cross-linking reaction with CMCNa, this substance turns into a slightly toxic urea derivative, which must be removed during the washing step, thereby further increasing the costs and the complexity of the production process. These drawbacks are extremely unfavourable, particularly in connection with those applications which require large scale production of the polymer hydrogels and which, consequently, involve high costs both with respect to the purchase of the starting materials and with respect to the disposal of the toxic substances which are produced during synthesis.
Furthermore, the formation of substances having a certain degree of toxicity, although very low, is a key factor for ruling out the possibility of using such polymers in biomedical and pharmaceutical applications.